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7
MCQ (Single Correct Answer)

In a nuclear reactor, moderators slow down the neutrons which come out in a fission process. The moderator used have light nuclei. Heavy nuclei will not serve the purpose, because

A
they will break up
B
elastic collision of neutrons with heavy nuclei will not slow them down
C
the net weight of the reactor would be unbearably high
D
substances with heavy nuclei do not occur in liquid or gaseous state at room temperature
8
MCQ (Multiple Correct Answer)

Fusion processes, like combining two deuterons to form a He nucleus are impossible at ordinary temperatures and pressure. The reasons for this can be traced to the fact

A
nuclear forces have short range
B
nuclei are positively charged
C
the original nuclei must be completely ionized before fusion can take place
D
the original nuclei must first break up before combining with each other
9
MCQ (Multiple Correct Answer)

Samples of two radioactive nuclides $A$ and $B$ are taken $\lambda_A$ and $\lambda_B$ are the disintegration constants of $A$ and $B$ respectively. In which of the following cases, the two samples can simultaneously have the same decay rate at any time?

A
Initial rate of decay of $A$ is twice the initial rate of decay of $B$ and $\lambda_A=\lambda_B$
B
Initial rate of decay of $A$ is twice the initial rate of decay of $B$ and $\lambda_A>\lambda_B$
C
Initial rate of decay of $B$ is twice the initial rate of decay of $A$ and $\lambda_A>\lambda_B$
D
Initial rate of decay of $B$ is same as the rate of decay of $A$ at $t=2 \mathrm{~h}$ and $\lambda_B<\lambda_A$
10
MCQ (Multiple Correct Answer)

The variation of decay rate of two radioactive samples $A$ and $B$ with time is shown in figure. Which of the following statements are true?

A
Decay constant of $A$ is greater than that of $B$, hence $A$ always decays faster than $B$
B
Decay constant of $B$ is greater than that of $A$ but its decay rate is always smaller than that of $A$
C
Decay constant of $A$ is greater than that of $B$ but it does not always decay faster than $B$
D
Decay constant of $B$ is smaller than that of $A$ but still its decay rate becomes equal to that of $A$ at a later instant
11
Subjective

$\mathrm{He}_2^3$ and $\mathrm{He}_1^3$ nuclei have the same mass number. Do they have the same binding energy?

Explanation

Nuclei $\mathrm{He}_2^3$ and $\mathrm{He}_1^3$ have the same mass number. $\mathrm{He}_2^3$ has two proton and one neutron. $\mathrm{He}_1^3$ has one proton and two neutron. The repulsive force between protons is missing in ${ }_1 \mathrm{He}^3$ so the binding energy of ${ }_1 \mathrm{He}^3$ is greater than that of ${ }_2 \mathrm{He}^3$.

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